The present invention relates to a Low Power Flux-gate Magnetometer for measuring the presence of magnetic fields.
Flux-gate Magnetometers are well known in the art. Such devices measure the strength of external magnetic fields by measuring changes in the inductance of a saturable-core inductor, often referred to as a flux-gate. In such devices the flux-gate inductor is driven by an alternating current signal having for example, a sinusoidal or triangular waveform. The AC input current induces an alternating magnetic field within the flux-gate core. The input signal has sufficient amplitude such that the induced current is large enough to drive the flux-gate core into saturation with each cycle of the input waveform. External magnetic fields are detected and measured by measuring changes to the inductance of the flux-gate coil resulting from stray magnetic flux associated with an external magnetic field.
When the flux-gate core becomes magnetically saturated, the magnetic permeability of the core drops toward unity, and the inductance of the flux-gate coil drops to only a fraction of its original value. The rapid decrease in inductance causes a corresponding drop in voltage as measured across the flux-gate inductor. Thus, by monitoring the voltage signal across the flux-gate inductor, the point in time when the magnetic flux-density within the flux-gate core reaches saturation can be determined in relation to the alternating cycle of the input waveform.
The magnetic flux density within the flux-gate core is a function of both the induced current flowing through the flux gate inductor and any stray magnetic flux associated with the presence of an external magnetic field. Since the external magnetic field component is variable, the saturation current I.sub.SAT necessary to drive the flux-gate core into saturation will vary depending on the strength and direction of the external magnetic field. Also, since the voltage waveform across the flux-gate inductor collapses when the flux-gate core reaches saturation, the actual saturation current, I.sub.SAT, which drives the flux-gate core into saturation can be determined by comparing the output voltage waveform to the input current waveform, and measuring the delay between the rise in the input current waveform and the collapse of the output voltage waveform. Based on these measured changes in the saturation current, the magnitude and direction of the external magnetic field can be derived.
Prior art flux-gate magnetometers are constant amplitude, alternating current devices. In other words, current is flowing through the flux-gate inductor throughout each cycle of the input voltage waveform. As noted, the magnitude of the saturation current is derived by monitoring the timing of the collapse of the voltage waveform across the flux-gate inductor as the flux-gate core reaches saturation. In the past this has typically been accomplished by placing a resistor in series with the flux-gate input, and grounding the flux-gate output. The series resistance is selected to be many times larger than the reactance of the flux-gate inductor such that when the circuit is fed by a voltage waveform, the current through the circuit is determined mainly by the resistor rather than the inductance of the flux-gate coil. The input to the flux-gate coil is also connected to one input of a voltage comparator which monitors the voltage across the flux-gate inductor. In this arrangement, alternating current continually flows through the resistor and flux-gate combination, and therefore, power is continually dissipated across the resistor.
The voltage across the flux-gate inductor generally follows the inductor current signal, but is advanced by 90.degree.. In general, the magnetometer circuit is driven by a sinusoidal waveform having sufficient amplitude to drive the flux-gate coil into saturation during each half cycle. As already noted, when the current through the flux-gate reaches saturation, the inductance of the coil drops drastically such that the voltage across the flux gate drops to approximately 0V while the flux-gate core remains saturated. The voltage across the flux-gate will remain approximately equal to zero as long as the flux-gate core remains saturated. However, since the flux gate is not a perfect inductor, parasitic resistance and inductance within the coil will cause the flux-gate voltage to have a slight slope while the flux-gate core is saturated, and a definite zero crossing can be ascertained. This zero crossing is detected by the comparator connected to the input of the flux-gate. From the timing of the zero crossings relative to the input signal, the magnitude of the saturation current can be ascertained. Additional circuitry compares the output of the comparator against the input voltage waveform to determine the relationship between the zero crossings and the input voltage signal. Since variations in the external magnetic field will alter the saturation current, the collapse in the voltage waveform, and thus the zero crossings detected by the comparator, will occur at different times relative to the input waveform, depending on the magnitude and direction of the external field. By comparing the comparator output signal against the input signal, the magnitude and direction of the external magnetic field can be derived.
As described, prior art flux-gate magnetometers provide an extremely linear, accurate and noise immune measure of magnetic fields. The operating characteristics of flux-gate magnetometers are very favorable, especially when compared to Hall effect and magnetoresistive devices. However, a significant drawback to such devices is their relatively large power consumption. Since the magnetometer is a constant current device, power is continually dissipated by the device. Furthermore, the current supplied to device must be sufficient to saturate the flux-gate core, typically this is on the order of tens of milliamps. This can be a significant drawback for some applications where low power consumption is a significant design criterion. For example, in applications where the magnetometer is to be battery powered, the current consumption of the device must be kept to a minimum in order to conserve battery life. Therefore, it is desirable to provide a magnetometer having the positive characteristics of traditional flux-gate magnetometers, including linearity, accuracy and noise immunity, while simultaneously drawing only a negligible amount of current.